Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $60.58   12.874%   $68.3791   $68.38   $68.40   $68.40  
  Purchase #2 $88.56   9.592%   $97.0547   $97.05   $97.05   $97.10  
  Purchase #3 $249.49   10.520%   $275.7363   $275.74   $275.75   $275.70  
  Purchase #4 $190.86   13.838%   $217.2712   $217.27   $217.25   $217.30  
  Purchase #5 $237.28   13.478%   $269.2606   $269.26   $269.25   $269.30  
  Purchase #6 $173.30   3.502%   $179.369   $179.37   $179.35   $179.40  
  Purchase #7 $211.50   3.267%   $218.4097   $218.41   $218.40   $218.40  
  7 purchase totals = $1,211.57       $1,325.4806   $1,325.48   $1,325.45   $1,325.60